output the command line called by subprocess?

It depends on the version of Python you are using. In Python3.3, the arg is saved in proc.args:

proc = subprocess.Popen(....)
print("the commandline is {}".format(proc.args))

In Python2.7, the args not saved, it is just passed on to other functions like _execute_child. So, in that case, the best way to get the command line is to save it when you have it:

proc = subprocess.Popen(shlex.split(cmd))
print "the commandline is %s" % cmd

Note that if you have the list of arguments (such as the type of thing returned by shlex.split(cmd), then you can recover the command-line string, cmd using the undocumented function subprocess.list2cmdline:

In [14]: import subprocess

In [15]: import shlex

In [16]: cmd = 'foo -a -b --bar baz'

In [17]: shlex.split(cmd)
Out[17]: ['foo', '-a', '-b', '--bar', 'baz']

In [18]: subprocess.list2cmdline(['foo', '-a', '-b', '--bar', 'baz'])
Out[19]: 'foo -a -b --bar baz'